This invention relates to an arrangement for adjusting the amplitudes of stereophonic left-plus-right and left-minus-right audio signals in response to the field-of-view condition of a zoom lens of a television camera viewing the scene from which the audio signals are derived.
Commercial television has until the recent past been monophonic or monaural in nature; i.e., only one audio channel was provided to accompany the video picture. The monophonic television has been well accepted. More recently, synthesized-stereophonic receivers have been introduced to the marketplace. These synthesized-stereo-audio receivers separate various frequency components of a monophonic audio signal and apply them differently to a pair of speakers associated with a television receiver in order to simulate stereophonic operation. Full or true stereophonic audio transmission standards have recently been adopted by the Broadcast Television Systems Committee of the Electronic Industries Association (EIA). According to these standards, two channels of audio are broadcast together with the television video. One audio channel is a baseband signal in the form of an audio sum signal representing the sum of the left and right audio signals (L+R) and corresponds to the convention monophonic signal. The second audio channel representing the audio difference signal (L-R) is AM double-side-band modulated onto a suppressed carrier having a frequency of twice the horizontal line frequency. The BTSC standard also provides that the L-R signal be compressed or companded for noise control.
FIG. 1 is a plan view illustrating an arrangement for generating television signals including stereophonic audio signals. FIG. 1 represents a scene which includes a stage 10 on which are arrayed a plurality of chairs 12, 14-16. A left microphone 20 and a right microphone 22 are placed near the left and right of the stage (as viewed facing the stage) and a television camera 24 is mounted to view the stage. The signals from the microphones are illustrated as being coupled over cables 26 and 28 and applied to a block 32 representing a matrix for generating L+R and L-R signals from the L and R audio signals. These L+R and L-R are in turn illustrated as being applied together with video signals from camera 24 to a modulator and transmitter illustrated as a block 34 for modulation and application to a broadcast antenna 36 for broadcast to television receivers, one of which is designated generally as 48. Receiver 48 is fitted with a viewing screen 50 and left and right loudspeakers 52 and 54, respectively, and a processing system arranged in known fashion for demodulating the received television signal to produce L+R and L-R signals and for decoding the L+R and L-R signals to produce left and right signals from loudspeakers 52 and 54, respectively. The processing system of receiver 48 produces the same signal from each of loudspeakers 52 and 54 when only a L+R signal is decoded.
The problem to which the invention is directed can be explained as follows. Camera 24 has a zoom lens 60. When the camera is in the position represented by solid lines, the zoom lens can be arranged for a wide viewing angle, so as to view the entire stage including chairs 12 through 16 and the human speakers (not shown) seated therein. Speakers near left microphone 20 produce L audio signals in microphone 20 to the substantial exclusion of R signals in right microphone 22, and likewise speakers near chair 16 activate right microphone 22 to produce R signals to the substantial exclusion of L signals. Speakers near the center of stage 10 cause activation of both microphones 20 and 22. A television viewer viewing receiver 48 displaying the scene with such a wide angle or panoramic view can see the entire stage, and the voices coming from loudspeakers 52 and 54 correspond generally to the locations of the various speakers on stage 10. However, in many cases camera 24 will be swiveled to a position such as shown by dotted lines and the zoom lens will be adjusted to a narrow viewing angle to focus onto the face of a speaker occupying chair 12. Thus, that speaker's face fills the screen 50. The viewer of screen 50 expects under those circumstances that the sound will come equally from loudspeakers 51 and 54 because the speaker is centered in viewing screen 50. However, because only left microphone 20 is near chair 12, the sound produced at the receiver will undesirably come chiefly from loudspeaker 52.